In the testing of electrical equipment, it is often necessary to characterize non-linearities of devices and circuits. For this purpose, it is useful to have a high accuracy source of a waveform containing equal amplitude components of multiple frequencies. For this purpose, the term "dual-tone" signal has come to be accepted in the art as a waveform which has multiple (e.g. not limited to two) equal amplitude sinusoidal components of differing frequencies. However, it is not signifigant to the practice or the use of the invention that the convention of sinusoidal waveforms or equal amplitudes of the plural components be observed and the plural components can be relatively attenuated or initially generated at any amplitude or with any desired waveshape. Therefore, a "dual-tone signal" merely connotes a signal having components comprising at least two waveforms. It is in this sense that the term "dual-tone" will be used throughout the following detailed disclosure of the invention to refer to both the signal and the apparatus for producing the same.
Numerous techniques of generation of waveforms containing plural frequency components have been used in the past with varying degrees of waveform accuracy and ease of use. For instance, analog waveform generation has often been used in the past, but is subject to drift, both in frequency and amplitude. The accuracy obtainable through analog waveform generation is simply not adequate for testing of modern electronic devices. The difficulty of developing arbitrary waveforms with oscillators used by analog waveform generators is also substantial.
Also among known techniques is the record/playback method which is of use where the circuit is required to handle a specific waveform in a predetermined way. However, this technique permits virtually no flexibility in waveform or frequency change without repeating the recording process. Digital recording techniques have allowed the record/playback technique of waveform generation to remain a viable waveform generation method for certain, extremely limited, applications in testing modern electronic devices. However, limitations are imposed by the periodicity of the recording medium, such as an optical disc.
Another method which can be digitally implemented and provides relatively high precision while permitting generation of waveforms which are arbitrary is that of computation of a waveform based on the use of a single fundamental pulse waveform. Typically, this fundamental waveform is obtained from an isolated read head transition, with respect to a given recording medium, which is then filtered and subjected to automatic gain control and thereafter used as a basic building block of any desired waveform. While this technique yields high flexibility and amplitude, phase and frequency resolution, it also requires a substantial computational load and produces only a single, though potentially complex, waveform. It is, therefore, less suitable for a test instrument since it requires each stand-alone instrument to possess such computational power. Further, in such an arrangement, transient-free switching from one waveform to another is not easily accomplished. Coherent generation of plural frequency components is also difficult and settling time is extended.
Therefore, for test instruments, direct digital waveform synthesis and synthesizers using this technique, both hereinafter sometimes referred to as "DDS", are most often employed. Digital waveform synthesizers are well known and are of widespread use in test equipment and as reference waveform sources. In modern communication and other electronic equipment, it has been necessary to develop such synthesizers which are capable of producing waveforms over a wide range of frequencies, typically 0.1 Hz to several MHz, and of high frequency resolution, typically 0.01 Hz. Since it is common in the art to specify a desired frequency as a decimal number, it has become almost a standard in the industry to use a decimally related reference frequency source, typically 10 MHz (for which high accuracy oscillators are readily available), for direct digital synthesizers. Also because of this convention, it is common to use a binary coded decimal (BCD) adder to accumulate the phase of the synthesized signal to produce the desired frequency from the reference frequency. The arrangement disclosed in U.S. Pat. No. 3,735,269 by Jackson is typical of such arrangements.
A block diagram of a common configuration DDS 10 is shown in FIG. 1. This block diagram is typical of either a decimal or binary DDS. The DDS has, as its first stage, a phase accumulator 11, a possible configuration for which comprises an adder 12 which has both clock and phase increment inputs and a latch 13, which receives inputs from a reference clock signal source, f.sub.c, and another means such as a switch or a register, which specifies the integer phase increment, N. This phase accumulator is typically an accumulating adder or counter or other means for generating a numeric sequence representing phase presettable to an arbitrary number N which generates addresses which are then applied to a phase to amplitude mapping or conversion device 14, such as a wave table, waveform memory or look up table, to obtain amplitude values for the waveform at instantaneous phase locations. Alternatively, other arrangements such as mapping or logic devices or any device capable of performing phase to amplitude conversion or mapping can be used. The output of the memory is latched at 15 in synchronism with the input reference frequency and applied to a digital-to-analog converter 16. The output of the D/A converter is then filtered to remove spurious (e.g. anti-aliasing, sampling noise, etc.) frequency components higher than the Nyquist frequency and, thus, produce the desired waveform at the desired frequency. Operation of this filter is not germane to an understanding of the invention and it is, therefore, not illustrated or further discussed. This arrangement is well understood in the art and further detailed disclosure is not deemed necessary. A review of the operation of such devices and a detailed disclosure of a decimally-operating DDS, implemented with binary hardware, is provided in U.S. application Ser. No. 07/325,359, filed Mar. 17, 1989, entitled Coherent Direct Digital Synthesizer, by the inventor herein and commonly assigned, which is hereby incorporated by reference herein.
As pointed out above, dual-tone sinusoidal signals are commonly used to characterize non-linearities of electronic circuits. For instance, whether the modulation is desired or parasitic, the cross-modulation product of a circuit could be analyzed by application of a dual-tone signal (in this case, containing two sinusoidal components at different frequencies). Amplifiers, adders and modulators as well as many other types of circuit can be analyzed or tested by exciting their inputs with a dual-tone signal and measuring the harmonics in order to characterize the non-linearity of the device.